An electrophotographic method has been utilized widely which supplies an area to be developed with toner by forming a magnetic brush with magnetic carriers along the magnetic lines of force of magnet installed in a developing sleeve to develop electrostatic latent images using 2-component developer that consists of toner and magnetic carrier.
For the typical contact development, developing characteristics depend significantly on conditions of the formed magnetic brush. Strong magnetic force acting among magnetic carriers causes a "hard" magnetic brush to be formed. Thus, toner already used for development is subjected to force produced by the magnetic brush and is moved, degrading toner quality and making it impossible to provide correct electrostatic. latent images. In consequence, black images may be scratched by the magnetic brush and fine lines may get dull, resulting in lowered resolution. To eliminate such defect, a method of forming a "soft" magnetic brush using a magnetic distributed carrier (hereinafter called the "distributed carrier") to reduce the scratching force has been proposed. The distributed carrier consists of particles with a particle diameter of 30 micrometers obtained by kneading magnetic grains such as magnetite with resin and crushing them.
The distributed carrier, however, may not be formed in a definite shape and its surface tends to be rough. So, the fluidity of developer consisting of carrier and toner becomes low, leading to the carrier not mixed with the toner sufficiently and preventing frictional chargeability of the developer. Further, since the magnetic particles tend to be exposed on the surface, it prevents the frictional chargeability of the surface of the carrier particles from being active through their friction. This causes such troubles as the toner cannot sufficiently electrified, resulting in troubles such as toner scattering and fogs. In addition, iron powder and magnetite particles used widely as magnetic material have low electric resistance; so, distribution of these materials in the resin often results in insufficient insulation of carrier particles, resulting in insufficient electrification of the toner, and development by the carrier particles themselves (i.e., carrier adhesion) due to charges induced by electrostatic latent images.
On the other hand, a process which keeps an electrostatic image-carrying member and the developer layer separate from each other and applies an AC field for development requires a thin, uniform, short-turfed developer layer to be formed. FIG. 2 shows a model of the non-contact developing process, wherein 1 denotes the latent image-carrying member, 2 denotes the developer, 3 denotes a developing sleeve, 4 denotes a magnet roller, 5 denotes an AC power source (2 kHz, 1 kVp-p), 6 denotes a DC power source (200 V), 7 denotes an agitating blade, and 8 denotes a regulating blade.
Unlike the contact development, in the non-contact development, toner already used for development is not scratched through friction with the magnetic brush. Thus, the non-contact development has the capability to provide high-quality latent images. Further, on the toner image already formed by development, different-color toner can easily be superimposed. So, the non-contact development method can advantageously be applied to color development. In particular, for the two-component, non-contact development, as the toner contains no magnetic particles such as iron grains and magnetite, clear color image can be obtained.
Obtaining high-resolution images in the non-contact development requires a developing gap (distance between carrying member and sleeve 3) to be maintained to 1 mm or less and a developer layer having a thickness of 1 mm or less to be formed. Further, as a less amount of developer passes through the developing area, a very dense developer layer must be formed to get high image density.
Forming a dense developer layer requires enhancing toner fluidity to increase toner density on the developing sleeve and making the toner more live. But such requirements have not been discussed fully.
Besides, during inversion for making toner adhere onto an exposed portion, carriers tend to adhere onto a non-exposed portion, resulting in production of fogs.
A Japanese Patent O.P.I. Publication No. 59-154469, discloses magnetic toner as developer for non-contact, inverting development. The magnetic toner comprises polystyrene-system resin and magnetic particles such as magnetite distributed therein. (The magnetic toner shall be hereinafter called a distributed carrier.) Use of such distributed carrier, however, limits the content of magnetic material in the carrier to no more than 80 weight percent. So, it is difficult to produce carriers having high magnetic force. Thus, magnetic force operating on the magnet roller arranged in the developing sleeve does not grow sufficiently high, which makes it impossible to prevent the carrier from adhering onto the non-exposed portion. In the reversal development, as described later on the basis of FIG. 7, the potential of the non-exposed portion is negative with respect to a developing bias. Therefore, the positive carrier particles tend to be attracted by the negative field, adhering onto the non-exposed portion.
Part of the distributed carrier particles that have adhered on the non-exposed portion appear on a duplicated picture as fogs.
FIG. 5 shows a sample model of surface potential of a latent image-carrying member (photoconductive material with an photoconductive layer consisting of organic optical conductors (OPC)) appearing when electrostatic latent images are formed by subjecting the latent image-carrying member to laser beams. Prior to being subjected to laser beams, the latent image-carrying member has been charged in -700 volts by a charger.
The surface potential of the non-exposed portion DA not subjected to laser beams is kept at -700 volts, while the surface potential of the exposed portion PH subjected to laser beams is -100 volts.
For reversal development, toner T charged negatively is caused to adhere onto the exposed portion PH with a developing bias of -600 volts applied. The surface potential of the latent image-carrying member is as shown in FIG. 7. As the toner T is charged by its friction with carrier particles, the toner T negatively charged causes the carrier particles to be charged positively. As described above, since the magnetic force of the carrier C is not sufficiently high, it is released from the developing sleeve, adhering onto the non-exposed portion DA.
Preventing this adhesion requires increasing sufficiently force of magnetically restraining the carrier on the sleeve. For this, carrier particles are made to be larger to increase such force operating on them. Larger carrier particles, however, result in a rough magnetic brush on the sleeve and higher turfs, which come in contact with the latent image-carrying member. Thus, contrary to expectation, more carrier particles tend to adhere onto the non-exposed portion DA.
Further, larger carrier particles prevent toner concentration from being increased sufficiently. For example, use of a distributed carrier whose particle diameter is about 20 micro-meters results in no toner being scattered, even though the toner concentration is 10 to 30 weight percent. With increase of particle diameter, however, the carrier surface area is decreased. e.g., for a particle diameter of 80 micrometers, the upper limit of tonre concentration is about 10 weight percent. In that case, as sufficient toner is not supplied to the developing area, sufficient image concentration may not be obtained.
Besides, for the distributed carrier, magnetic particles are likely to present on the surfaces of carrier particles; so, its electric resistance cannot be increased sufficiently. If a carrier whose resistance value is not sufficiently high is used, charges having the polarity reverse to that of charges produced on the latent image-carrying member are induced on carrier particles by electrostatic induction, and such carrier particles are inclined to adhere onto the exposed portion PH (see FIG. 7) of tahe latent image-carrying member along with the toner. As the carrier particles are larger than the toner particles, the applied carrier particles lower the resolution. For color development, the developed color may get turbid due to adhesion of black and/or brown carrier particles as well as color toner. This is a critical disadvantage.
This phenomena tends to appear in non-contact development wherein an AC bias applies periodic high bias voltage.
As the diameters of the distributed carrier particles range from 10 to 30 micro-meters, the carrier fluidity is low. So, the fluidity of the developer consisting of the carrier and toner is also lowered. In consequence, carrier particles and toner are not frictionally charged fully, resulting in toner scattering, fogs and/or rough images.